In the last 15 years, the field of molecular electronics has developed rapidly with the discovery of organic conductive and semiconductive compounds. In this time, a multitude of compounds which have semiconductive or electrooptical properties has been found. It is the general understanding that molecular electronics will not displace conventional semiconductor units based on silicon. Instead, it is assumed that molecular electronic components will open up new fields of use in which suitability for coating large areas, structural flexibility, processability at low temperatures and low costs are required. Semiconductive organic compounds are currently being developed for fields of application such as organic field-effect transistors (OFETs), organic luminescent diodes (OLEDs), sensors and photovoltaic elements. Simple structuring and integration of OFETs into integrated organic semiconductor circuits makes possible inexpensive solutions for smart cards or price tags, which have not been realizable to date with the aid of silicon technology owing to the cost and the lack of flexibility of the silicon units. It would likewise be possible to use OFETs as switching elements in large-scale flexible matrix displays.
All compounds have continuous conjugated units and are divided into conjugated polymers and conjugated oligomers according to the molecular weight and structure. Oligomers are generally distinguished from polymers in that oligomers usually have a narrow molecular weight distribution and a molecular weight up to about 10 000 g/mol (Da), whereas polymers generally have a correspondingly higher molecular weight and a broader molecular weight distribution. However, it is more sensible to distinguish by the number of repeat units, since a monomer unit can quite possibly reach a molecular weight of 300 to 500 g/mol, as, for example, in the case of (3,3″″-dihexyl)quarterthiophene. In the case of a distinction by the number of repeat units, reference is still made to oligomers in the range of 2 to about 20. However, there is a fluid transition between oligomers and polymers. Often, the difference in the processing of these compounds is also expressed with the distinction between oligomers and polymers. Oligomers are frequently evaporable and can be applied to substrates by means of vapour deposition processes. Irrespective of their molecular structure, polymers frequently refer to compounds which are no longer evaporable and are therefore generally applied by means of other processes.
An important prerequisite for the production of high-value organic semiconductor circuits is compounds of extremely high purity. In semiconductors, order phenomena play an important role. Hindrance of uniform alignment of the compounds and development of particle boundaries lead to a dramatic decline in the semiconductor properties, such that organic semiconductor circuits which have been constructed using compounds not of extremely high purity are generally unusable. Remaining impurities can, for example, inject charges into the semiconductive compound (“doping”) and hence lower the on/off ratio or serve as charge traps and hence drastically lower the mobility. In addition, impurities can initiate the reaction of the semiconductive compounds with oxygen, and oxidizing impurities can oxidize the semiconductive compounds and hence shorten possible storage, processing and operating times.
The most important semiconductive poly- or oligomers include the oligothiophenes whose monomer unit is, for example, 3-hexylthiophene. In the linkage of individual or plural thiophene units to form a polymer or oligomer, it is necessary in principle to distinguish two processes—the single coupling reaction and the multiple coupling reaction in the sense of a polymerization mechanism.
In the single coupling reaction, generally two thiophene derivatives with identical or different structure are coupled with one another in one step, so as to form a molecule which then consists of in each case one unit of the two starting materials. After a removal, purification and another functionalization, this new molecule may in turn serve as a monomer and thus open up access to longer-chain molecules. This process leads generally to exactly one oligomer, the target molecule, and hence to a product with no molar mass distribution and a low level of by-products. It also offers the possibility of building up very defined block copolymers through the use of different starting materials. A disadvantage here is that molecules which consist of more than 2 monomer units can be prepared only in a very complicated manner merely owing to the purification steps and the economic outlay can be justified only for processes with very high quality demands on the product.
One process for synthesizing oligothiophenes is described in EP 1 026 138. In the actual polymerization, a regioselectively prepared Grignard compound is used as the monomer (X=halogen, R=substituent):

For the polymerization, the polymerization in a catalysis cycle is started by the Kumada method (cross-coupling metathesis reaction) with the aid of a nickel catalyst (preferably Ni(dppp)Cl2).

The polymers are generally obtained in the necessary purity via Soxhlet purifications.
In EP 1 026 138, the reaction is effected in such a way that first (as quantitatively as possible) the Grignard reaction is prepared and then the thiophene is polymerized with C—C bond formation by adding the nickel catalyst. Similar processes can be found, inter alia, in U.S. Pat. No. 4,521,589 and in Loewe et. al., Advanced Materials 1999, 11, No. 3, p. 250-253 and Iraqi et al., Journal of Materials Chemistry, 1998, 8 (1), p. 25-29.
The reaction mechanism of the “Kumada” reaction is, according to current opinion, that of a so-called “quasi-living” Grignard metathesis reaction. On this subject, reference is made to two documents by Richard D. McCullough et al, specifically M. C. Iovy et al. Macromolecules 2005, 38, 8649-8656, and E. E. Sheina, Macromolecules 2004, 37, 3526-3528.
According to these, the polymerization, especially of 3-substituted 2,5-dibromothiophenes, is thought to proceed by the following mechanism (scheme 1, from Macromolecules 2005, 38, 8649-8656):

Thus, according to this mechanism, the chain length depends very substantially on the number of active catalyst sites and the amount of monomer, the molar mass being broadened by random distribution of the monomer between the growing chains. The growing chain is coordinated predominantly on a nickel site.
The connection between molar mass and catalyst concentration also becomes clear from FIG. 3 of the same document: